Retroviruses and retroposons

Helper virus provides functions absent from
a defective virus, enabling the latter to complete the infective cycle
during a mixed infection.

Figure 16.10 Replication-defective transforming
viruses have a cellular sequence substituted for part of the viral
sequence. The defective virus may replicate with the assistance of a
helper virus that carries the wild-type
functions.

Figure 16.11 Replication-defective viruses may be
generated through integration and deletion of a viral genome to generate a
fused viral-cellular transcript that is packaged with a normal RNA genome.
Nonhomologous recombination is necessary to generate the
replication-defective transforming genome.